Quantum nano-structure semiconductor laser

ABSTRACT

On a grooved semiconductor substrate having a plurality of V-grooves individually extended in directions perpendicular to a direction Is of advance of an oscillated laser beam and mutually disposed in parallel along the direction Is of advance of the laser beam, a plurality of quantum wires ( 11 ) are formed on the V-grooves by selective growth of a Group III-V compound. The plurality of quantum wires are adapted to serve as limited-length active layer regions mutually disposed in parallel along the direction Is of advance of the laser beam with a period of an integer times of a quarter wavelength in a medium of a laser active layer and individually corresponding to stripe widths of laser. Consequently, a quantum nano-structure semiconductor laser satisfying at least one, or preferably both, of the decrease of a threshold and the stabilization of an oscillation frequency as compared with a conventional countertype can be provided.

TECHNICAL FIELD

This invention relates to a quantum nano-structure semiconductor laserthat is particularly effective for application to a distributed feedback(DFB) semiconductor laser and to improvements in and concerning aquantum nano-structure array applicable significantly to variousoptically functioning devices.

BACKGROUND ART

In the semiconductor lasers, the distributed feedback semiconductorlaser that is provided in the direction of advance of a waveguide withperiodic structures for refractivity and gain will discharge a veryimportant role as a device for use in the future Wavelength DivisionMultiplexing (WDM) operation in respect that it is capable ofcontrolling an oscillating wavelength exactly and it facilitatesintegration because it obviates the necessity of a step of cleavageindispensable as in the Fabry-Perot laser.

The first problem that is encountered by the distributed feedbacksemiconductor laser pertains to simplification of the process ofproduction thereof In the initial stage of its development, it wascustomary to form a lower clad layer, a lower guide layer, an activelayer and an upper guide layer from a substrate upward in the firstcycle of the process of crystal growth, curve gratings conforming to thewavelength in a waveguide on the upper guide layer, and form an upperclad layer on a guide layer having a periodic structure in the secondcycle of the crystal growth (refer, for example, to Document 1: P. K.York, J. C. Connolly et al, “MOCVD regrowth over GaAs/AlGaAs gratingsfor high power long-lived InGaAs/AlGaAs lasers,” Journal of CrystalGrowth 124 (1992) 709-715).

Further, for the purpose of securing the confinement of carriers andoptical field in the lateral direction, stripes are formed along anoptical waveguide by means of wet etching through a silicon dioxidemark, and lateral surfaces of the stripes are embedded with a currentblocking layer and an ohmic contact layer by the third cycle of growth.It is often the case that the fourth cycle of growth is necessary toflatten the device surface after removing the silicon dioxide mark usedfor the third selective growth.

Such multiple cycles of lithography and crystal growth results inboosting the cost of production and impeding the dissemination of theproduct in industry. Further, since the re-growth interface exists inthe neighborhood of the active layer, it causes an additional leak pathof the drive current and increases the threshold current. The techniqueof this nature, therefore, has its limits in terms of principle andcannot continue to be similarly useful in the future.

In contrast, the quantum nano-structure semiconductor laser usingquantum wires and quantum dots in the active domain has been found topossess various great merits besides the quantum effect initiallyexpected. In the case of the quantum dots, for example, particularlysince the dots are embedded with a material of a broad band gap, theycan confine therein a carrier and, by virtue of a simple alteration ofthe conditions for crystal growth, can aim at attaining an addition tothe function without entailing an increase in the cost of production.The merit of this nature suggests the possibility of proving extremelyadvantageous for the construction of semiconductor lasers that will findutility fiber to home age to arrive shortly.

The present inventors, therefore, have hitherto studied processes forthe production of quantum wires and have searched stepwise for newprocesses to redue the device production cost while maintaining thedevice performance. Generally, the quantum nano-structure such as aquantum wire or quantum dots with a narrow band gap material embedded ina wide band gap material to form a clad layer in a size of several nmequaling the de Broglie wavelength of an electron has a density ofstress concentrated on a specific energy level and, therefore, befitsthe realization of a high performance optical devices. Even for the sakeof clearing the problem regarding a process complexity of deviceproduction, therefore, it is more rational to aim at realizing such aquantum nanostructure configuration. That is, in having quantum wiresintegrated at a high density in a positional relation of a specificregularity, it is ideal to realize this integration by one time crystalgrowth. The realization of this integration results in rationalrealization of a semiconductor laser allowing control of wavelength anda supersaturated absorber necessary for self-starting oscillation of anultrahigh speed solid laser.

For the sake of confining light in a semiconductor waveguide, it isnecessary that the upper and lower clad layers should be formed at leastin a thickness in the approximate range of 0.5 to 1 μm. When a gratingis formed on a substrate and a lower clad layer of such a thickness isgrown and, even thereafter, the grating is allowed to retain asatisfactory shape on the surface of the lower clad layer, then it ismade possible to form an active layer approximating closely to thegrating of the clad layer by one time crystal growth and consequentlysimplify a process for the fabrication of a distributed feedbacksemiconductor laser prominently.

From this point of view, the present inventors have suggested a methodof forming falcate quantum wires by first forming stripe patterns in the(1-10) direction on a compound semiconductor substrate of the (100)azimuth by following the procedure reported in Document 2 (Xue-Lun Wanget al., “Fabrication of highly uniform AlGaAs/GaAs quantum wiresuperlattices by flow rate modulation epitaxy on V-grooved substrates,”Journal of Crystal Growth 171 (1997) 341-348), forming V-grooves by wetetching, growing thereon AlGaAs and InAlAs confining in the respectivecompositions Al, an element sparingly producing surface atomicmigration, thereby forming a clad layer retaining the profile ofV-shaped grooves, and then supplying GaAs and InGaAs confining Ga andIn, elements producing a large surface atomic migration. In this case,when (111)A planes are allowed to be formed as inclined planes tointersect each other, it is made possible to attain growth to athickness of 1 μm or more in the direction of thickness of growth whilethe shape of V-grooves mentioned above is retained satisfactorily bysetting a proper temperature of crystal growth so as to suit the crystalmixing ratio of the compound semiconductor.

The stationary growth profile is made possible when the growth rate ofthe (111)A plane having a lower rate of crystal growth against that ofthe (100) plane having a higher rate of growth is equal to sin θrelative to the inclination θ of the crystal plane. Generally, the rateof growth of a specific plane depends on the chemical activity of thatplane and the diffusion of the raw material elements from theenvironment, and the anisotropy due to the azimuth of a crystal planetends to fade and the rate of growth becomes uniform in accordance asthe temperature increases. The rate of growth declines in the azimuth ofinactive crystal such as the (111)A plane when the temperaturedecreases. By adjusting the temperature of the substrate, therefore, itis made possible to form grating profiles at a fixed period.

In this suggestion, however, the period of repetition, namely the pitchof parallel V-grooves, is restricted to the order of microns. This pitchproves unduly coarse as for the purpose of producing a distributedfeedback semiconductor laser of fully satisfactory characteristicproperties and requires further refinement to the order of submicrons.In the case of submicron gratings having the shortest possible periodrelative to the distance of diffusion of Ga atoms adhering to thesurface of the substrate, however, it is generally considered difficultto attain the necessary growth while a specific profile of crystalgrowth is retained. In fact, the growth was impossible at first.

Subsequently, the present inventors, as a result of further experimentsand studies, managed to succeed in satisfactorily retaining the profileof V-grooves even on the surface of an AlGaAs layer grown on asubstrate, though to a certain thickness.

This achievement is reported in Document 3 (C. S. Son, T. G. Kim, X. L.Wang and M. Ogura, “Constant growth of V-groove AlGaAs/GaAs multilayerson submicron gratings for complex optical devices,” J. Cryst, Growth,Vol. 221, No. 1/4, pp. 201-207 (December 2000)).

In finding the maximum film thickness of the AlGaAs layer formed on theGaAs substrate gratings while the profile of the gratings is infalliblyretained, a trial of alternate superposition of AlGaAs layers about 100nm in thickness and GaAs layers about 10 nm in thickness will facilitatedue judgment. Document 3 mentioned above inserts a report regardingtrial alternate superposition of a pair of an AlGaAs layer having arelatively large thickness of about 100 nm and a GaAs layer having arelatively small thickness of about 10 nm on a GaAs substrate havingV-grooves formed with a pitch of 0.38 μm on the surface thereof As aresult of this experiment, it was found that the profile of theV-grooves of the substrate was satisfactorily retained up to about 1 μmin thickness of superposed layers from the surface of the substrate. AGaAs quantum wire was formed in a falcate cross section and in parallelto the bottom parts of these V-grooves. According to the techniqueprevalent at the time of disclosure of this publication, however, theprofile of V-grooves was seriously impaired when the height ofsuperposed layers reached a level exceeding 1 μm.

Of course, in the actual manufacture such as of a distributed feedbacksemiconductor laser, though one AlGaAs layer suffices as a clad layerand one or more GaAs quantum wires laid in the vertical directionsuffice, it may be safely concluded that the lower V-grooves have abetter profile and the quantum wires formed in these V-grooves likewisehave a better cross-sectional shape in accordance as the upper V-groovesoffer more resistance to the collapse of profile. This fact proves thateven on the upper surface of a single AlGaAs clad layer formed in anarbitrary film thickness, gratings are enabled to retain a fullysatisfactory profile and implies that the quantum wires to be formedthereon are similarly satisfactory. Even an active layer appearing to bea quantum well layer of the shape of a continuous plane and not quantumwires, namely even an active layer of the shape of a fairly uniform flatplane (the shape of a sheet) having the thickness and width thereof notgeometrically modulated or corrugated in conformity with the period ofgratings of V-grooves, allows the periodic structure such as of thedistribution of refractivity, supposed that gratings of either a guidingor cladding layer underneath are constructed with such high accuracy asexpected, and can be similarly utilized very effectively as an activelayer in a distributed feedback type semiconductor layer. For the sakeof simplicity, the quantum wires will be exclusively described below.

The present inventors have further made studies and experiments with aview to enabling the V-grooves up to a greater thickness of superposedlayers to retain a good profile and consequently have succeeded inimproving the technique disclosed in Document 3 mentioned above to anextent of suggesting such conditions that even when the clad layer isformed in a thickness at least exceeding 1 μm, preferably approximatingclosely to or even surpassing 1.5 μm, the V-grooves formed on thesurface thereof may retain a fully satisfactory profile. The inventionperfected based on this knowledge has been already disclosed in JapanPatent Application No. 2000-404645 (JP-A 2002-204033).

In this patent document, a basic structure is reported to be obtained byetching a plurality of V-grooves extending in the [01-1] direction on a(100) GaAs substrate with a pitch of the order of submicrons in such amanner that the lateral surfaces thereof each constitute a (111)A plane,subjecting the V-grooves to a treatment for removal of a surface oxidelayer, thereby enabling the V-grooves to retain an angle of 80 degreesor less even after the treatment, and thermally cleaning them at atemperature in the range of 680° C. to 720° C., thereby forming on thesurface of the GaAs substrate a buffer layer of the same material GaAs.These treatments enable the apexes between the adjoining V-grooves whichhave been dulled by the thermal cleaning to be recovered, allow anAlGaAs layer having an Al percentage of 0.3 to 0.6 or an InAlAs layerhaving an In percentage of 0.05 to 0.3 to be grown as a clad layer, andfurther warrant supply of GaAs or InGaAs as well.

Further, the process of growing on the part forming quantum wires or aquantum well layer an AlGaAs guide layer having a smaller Al percentagethan the Al percentage of the AlGaAs layer constituting a clad layer oran InAlAs guide layer having a smaller In percentage than the Inpercentage of the InAlAs layer constituting a clad layer and growingfurther thereon as an upper side clad layer an AlGaAs layer having an Alpercentage of 0.3 to 0.6 or an InAlAs layer having an In percentage of0.05 to 0.3, is actually favorable for the manufacture of a deviceutilizing this invention.

By this technique, the V-grooves are enabled to retain a fullysatisfactory profile till the height of the laminated structuregenerously exceeds 1 μm and even reaches 1.5 μm. Of course, the factthat the profile of V-grooves can be retained to such a height provesthat the profile of quantum wires in the lower part and the profileinherent in the V-grooves are highly favorable. In fact, when thistechnique is embodied in quantum wires embedded in an active layer of adistributed feedback semiconductor laser, the quantum wires are found toacquire more than satisfactory characteristic properties. In short, theinvention of the aforementioned Japanese Patent Application hasestablished that even when an AlGaAs layer is grown as a single cladlayer to a thickness in the range mentioned above by way of anexperiment of constructing a laminate structure formed by repeating suchmultilayer films, the profile of gratings formed on the surface thereofmatch the substrate gratings and can be retained fully satisfactorily.The quantum wires that are formed thereon acquire fully satisfactoryprofile and characteristic properties as a matter of course. The AlGaAsclad layer having a thickness falling short of the upper limit of therange can be expected to bring still better results.

As regards the quantum wires, the present inventors' efforts havedeveloped such an environment as allows provision of quantum wires offairly better performance. The method of production thereof is simpleand capable of forming by one time selective growth of high-densitymultiple quantum wires with highly satisfactory profile andcharacteristic properties at a necessary position in the structure of agiven device. Such an excellent structure of quantum wires as this hasnot made a true contribution to the industry unless it has achieved adevelopment in terms of application.

This invention has been initiated with a view to developing suchapplications and, therefore, is aimed at providing a quantumnano-structure semiconductor laser particularly promising a growingdemand, which is capable of satisfying at least either or preferablyboth of the reduction of the threshold value and the stabilization offrequency of oscillation, the factors which are constantly in need ofimprovement. It is provided, however, that with the same intent, thisinvention is not limited to the quantum nano-structure semiconductorlaser but has as its intrinsic object the provision of a quantumnano-structure array which, uses periodically disposed limited-lengthquantum wires or quantum dots and which can be developed into variousoptically functioning devices.

DISCLOSURE OF THE INVENTION

For the purpose of accomplishing the object mentioned above, the firstaspect of this invention is directed toward providing a quantumnano-structure semiconductor laser comprising a grooved semiconductorsubstrate furnished with a plurality of V-grooves individually extendedin directions perpendicular to a direction of a laser beam and mutuallydisposed in parallel along the direction of the laser beam and aplurality of quantum wires formed one on each of the V-grooves byselective growth of a Group III-V compound, the plurality of quantumwires being mutually disposed in parallel along the direction of advanceof the laser beam with a period of an integer times of a quarterwavelength in a medium of a laser active layer and disposed individuallyas an active layer region of a limited length corresponding to a widthof stripes of laser. Thus, this invention is capable of providing aquantum nano-structure semiconductor laser that satisfies at least one,or preferably both, of the reduction of the threshold and thestabilization of the frequency of oscillation more satisfactorily thanthe conventional countertype.

It is provided, however, that the waveguide mode may be stabilized byintentionally varying the period of parallel disposition of the quantumwire array from the period of an integer times of a quarter wavelengthmentioned above instead of disposing the quantum wires in parallel alongthe direction of advance of a laser beam with a period of an integertimes of a quarter wavelength in the medium of the laser active layer.In this way, the oscillation of a broadband wavelength or theoscillation of short pulses in the state of mode lock may be attained bypromoting compensation of the dispersion between the oscillation modes.

A structure commendable from the viewpoint of material can be providedby this invention. Examples are a semiconductor laser which is providedwith V-grooves of a limited length formed in the [01-1] direction on aGaAs (100) or (311)A substrate, quantum wires of a limited length grownon the V-grooves of the limited length and made of GaAs or InGaAs and aclad region of GaAs or AlGaAs so disposed as to cover the quantum wires;and a semiconductor laser which is provided with V-grooves of a limitedlength formed in the [01-1] direction on an InP (100) or (311)Asubstrate, quantum wires of a limited length grown on the V-grooves ofthe limited length and made of InGaAs and a clad region of InAlAs soformed as to cover the quantum wires. These semiconductor lasers may beused as a gain-coupled or refractive index-coupled distributed feedbacklaser.

This invention also provides a structure which has adjoining quantumwires mutually connected in a flat part between the adjacent V-groovesor on an upwardly curved convex part of each of the V-grooves so as toassume eventually the shape of a plane (though slightly corrugated)instead of using mutually independent quantum wires. Also in this case,the material described above by way of example can be adopted.

Further, this invention provides a quantum nano-structure semiconductorlaser wherein a laser active layer of a prescribed width which is aneffective laser oscillating part formed by selective growth of a GroupIII-V compound on a semiconductor substrate is in the shape of a flatsheet devoid of corrugation and the periodic perturbation is provided atboth sides of the laser active layer in the direction perpendicular tothe laser beam from V-grooves individually extended in the directionsperpendicular to the laser beam and mutually disposed in parallel alongthe direction of the laser beam with a period of an integer times of thequarter wavelength in the medium of the laser active layer.

As another embodiment, this invention provides a quantum nano-structuresemiconductor laser which is provided with a grooved Group III-Vcompound semiconductor substrate furnished with a plurality of V-groovesindividually extended in the directions perpendicular to the directionof the laser beam and mutually disposed in parallel along the directionof the laser beam with a period of an integer times of the quarterwavelength in the medium of the laser active layer, a plurality ofInGaAs or InAs quantum dots formed in each valley part of the V-groovesby growing InGaAs or InAs to a thickness exceeding the critical filmthickness on the V-grooves, and the plurality of quantum dots serving aslaser active regions, and the GaAs or AlGaAs layers covering the regionsserving as clad regions.

Even when the V-groove structure and the quantum dots mentioned aboveare used, the waveguide mode may be stabilized by intentionallydisplacing the plurality of V-grooves from the period of an integertimes of the quarter wavelength instead of disposing them with a periodof an integer times of the quarter wavelength instead of disposing themwith a period of an integer times of the quarter wavelength in themedium of the laser active layer, or the oscillation at a broad bandwavelength or the short pulse oscillation in the mode lock operation maybe attained by promoting compensation of the dispersion between theoscillation modes.

Of course, similarly again, this invention contemplates providing suchsubstrate materials as are favorable for the application of quantumdots. It is commendable to adopt a GaAs (100) or (311)A substrate or anInP (100) or (311)A substrate as the grooved Group III-V compoundsubstrate.

Further, a distributed feedback semiconductor laser obtained bymesa-etching the lateral surfaces of the structure including quantumdots may be suggested and a semiconductor laser obtained by impartingcorrugation to the lateral surfaces of the structure including quantumdots which are parallel to the propagation direction of light, therebytransforming the structure to a distributed feedback type, may besuggested.

This invention further proposes a semiconductor laser which is obtainedby perforating a structure including quantum dots with a plurality ofthrough holes spaced with a certain period along the lateral sides ofthe structure and also at both end of the structure spaced perpendicularto the propagation direction of light in such a manner as to leavebehind the stripe parts for passing the laser beam. Here, the period ofspacing the holes is typically set at a half of the wavelength in themedium.

This invention can provide not only the quantum nano-structuresemiconductor laser described above but also a quantum nano-structurearray that is adaptable to a wide variety of optical functional devices.For example, there can be provided a quantum nano-structure array whichis provided with a grooved semiconductor substrate furnished with aplurality of V-grooves individually extended in the directionsperpendicular to the propagation direction of light and mutuallydisposed in parallel along the propagation direction of light, and witha plurality of quantum wires formed on the V-grooves by the selectivegrowth of a Group III-V compound being mutually disposed along the lightdirection with a period of an integer times of the quarter wavelength inthe medium of the waveguide layer for passing the light and individuallyadapted to serve as waveguide regions of a limited width correspondingto the width of the waveguide. Here again, the waveguide mode may bestabilized or the compensation of dispersion concerning the lightpassing the waveguide may be effected by intentionally displacing theplurality of quantum wires from the period of an integer times of thequarter wavelength mentioned above instead of disposing them along thedirection of light propagation with a period of an integer times of thequarter wavelength in the medium of the waveguide layer.

By the same token, this invention can provide a quantum dot array whichis provided with a grooved semiconductor substrate furnished with aplurality of V-grooves individually extended in the directionsperpendicular to the light propagation and mutually disposed in parallelalong the light propagation with a period of an integer times of thequarter wavelength in the medium of the waveguide layer, the groovedsemiconductor substrate having a plurality of InGaAs or InAs quantumdots formed in the valley parts of the V-grooves by growing InGaAs orInAs to a thickness exceeding the critical film thickness on theV-grooves, and the plurality of quantum dots serving as laser activeregions, and the GaAs or AlGaAs layers covering the regions serving asclad regions. Again, similarly to the preceding case, the waveguide modemay be stabilized or the compensation of dispersion may be effected byintentionally displacing the plurality of quantum dots from the periodof an integer times of the quarter wavelength instead of disposing themwith a period of an integer times of the quarter wavelength in themedium of the waveguide layer.

This invention can provide a structure that is convenient for theconstruction of a various kinds of optical integrated circuits. Forexample, an array structure which is obtained by disposing a pluralityof quantum nano-structure semiconductor lasers contemplated by thisinvention as described above on one and the same substrate andconnecting them with a ridge type waveguide may be used not only as asemiconductor laser array but also as an integrated multi-wavelengthlight source, with the conditions of component parts or a ridge typewaveguide structure or relevant parameters duly varied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a schematic diagram of a semiconductor laser as the firstpreferred embodiment of this invention.

FIG. 1(B) is a schematic diagram of a modified example of thesemiconductor laser of this invention shown in FIG. 1(A).

FIG. 2(A) is a schematic diagram of another modified example of thesemiconductor laser of this invention shown in FIG. 1(A).

FIG. 2(B) is an explanatory diagram schematically illustrating oneportion of the active layer part of the semiconductor laser shown inFIG. 2(A).

FIG. 3 is a schematic diagram of yet another modified example of thesemiconductor laser of this invention shown in FIG. 1(A).

FIG. 4(A) is a schematic diagram taken in a cross section of asemiconductor laser as another preferred embodiment of this invention.

FIG. 4(B) is a schematic diagram in a cross section taken through FIG.4(A) along line 4B-4B.

FIG. 5 is a schematic diagram of a semiconductor laser as still anotherpreferred embodiment of this invention.

FIG. 6 is a schematic diagram of a semiconductor laser as a furtherpreferred embodiment of this invention.

FIG. 7 is a schematic diagram of a preferred embodiment of thisinvention in constructing a quantum nano-structure semiconductor laserarray or multi-wavelength light source.

BEST MODE FOR CARRYING OUT THE INVENTION

This invention will be described more specifically below with referenceto the drawing annexed hereto.

In FIG. 1, one example of the structure of a distributed feedbacksemiconductor laser 10 as one preferred embodiment of a quantumnano-structure semiconductor laser constructed in accordance with thisinvention. Basically, the whole structure is manufactured by the methodof production disclosed in Japan Patent Application No. 2000-404645(JP-A 2002-204033). It is provided with an active layer which, as viewedin the direction of light emission Is, comprises a V-groove substratehaving a plurality of V-grooves disposed in parallel thereon andlimited-length GaAs or InGaAs quantum wires 11 individually formed inV-groove regions of the V-groove substrate and typically measuring 5 to20 nm in thickness and 10 to 50 nm in the size in the direction of laserbeam emission Is and 0.5 to 2 μm in the size (width) of the directionperpendicular to the direction of laser beam emission Is. The pitch ofparallel disposition of the plurality of limited-length quantum wires 11along the direction of laser beam emission Is is set at nλ/4 (wherein ndenotes an integer) which is an integer times ¼ of the wavelength λ inthe medium such as, for example, a period of ¼ or ¾ wavelength in themedium (0.15 to 0.5 μm).

The perimeter of this array of quantum wires is covered with an upperand lower guide layer 13 about 0.2 μm in a vertical size, a lower AlGaAsclad layer 12 about 1 μm in thickness and an upper clad layer 14. One ofthe upper and lower clad layers 14 and 12 is of an N type and the otherlayer is of a P type. The cross section of these quantum wires 11, asdescribed in Japan Patent Application No. 2000-404645, generally has afalcate shape. The clad layers 12 and 14 maybe formed of GaAs.

To cite a slightly more concrete example of the procedure of production,a group of V-grooves of a limited length is repeatedly formed in the(1-10) direction on a (100) substrate with a period of an integer times¼ of the quarter wavelength in the medium, an N type AlGaAs lower cladlayer 12 having an Al percentage of about 0.4 and a non-doped AlGaAsguide layer 13 having an Al percentage of about 0.2, for example, aresequentially formed thereon, further InGaAs quantum wires 11 having anIn percentage of about 0.1 is formed, and subsequently a non-dope AlGaAsguide layer 13 having an Al percentage of about 0.2 (since this guidelayer, when formed, transforms into an integral member covering thequantum wires 11, the upper and lower guide layers are jointly denotedby a reference numeral 13 in the diagram) and a P type AlGaAs upper cladlayer 14 having an Al percentage of about 0.4 are continuously grown.

In this structure contemplated by this invention, the laser oscillationis accomplished with a lower threshold than ever because theperiodically arrayed quantum wires 11 are closed at both ends thereofand consequently are given a limited length and the diffusion ofminority carriers is suppressed in the lateral direction along thequantum wire. In fact, this achievement has been demonstrated by thepresent inventors' experiment.

Incidentally, the existing manufacturing technique has set examples offorming V-grooves in the [01-1] direction by using not only a GaAs (100)substrate but also a GaAs (311)A substrate. The GaAs or InGaAs quantumwires 11, therefore, can be formed on these substrates and the InGaAsquantum wires 11 of a limited length can be formed in the [01-1]direction by using an InP(100) or (311)A substrate. To the structuresthus obtained, the present invention can be applied. Particularly, whenthe InP substrate is used, the lower clad layer on the V-groovesubstrate is allowed to have a decreased thickness and the retention ofa periodic structure of V-groove array of a short period is facilitatedbecause the InP substrate is transparent to the oscillation wavelength.

Now, the other embodiments of this invention will be explained belowwith reference to the diagrams of FIG. 1(B) onward. In all the diagramsincluding and following FIG. 1(B), the reference numerals identical tothose used in FIG. 1(A) are meant to represent the identical or similarstructural elements. The repeated explanation of these elements will beomitted.

First, as briefly stated above, a structure, such as the semiconductorlaser 10A of this invention illustrated in FIG. 1(B), is conceivableeven though the quantum wires 11 are not formed mutually in a perfectlyindependent form and individually in a geometric form on the V-groovegrating. Specifically, even an active layer which appears to be aquantum well layer in the form of a continuous plane, as observed whenthe opposite sides in the direction of light emission of the parts 11corresponding to the quantum wires in FIG. 1(A) are connected to theparts 11 of the adjacent quantum wires through a thin active layer part11 a, is enabled to manifest the same function and effect as thesemiconductor laser 10 illustrated in FIG. 1(A) so long as the thicknessof an active layer 15 is modulated, as illustrated in FIG. 1(B), inconformity with the period of V-groove grating, namely the period ofnλ/4 (wherein n denotes an integer), such as ¼ or ¾, which is an integertimes of the quarter wavelength λ in the medium. This point has beendemonstrated by the present inventors' experiment. As depicted in thediagram, the parts 11 corresponding to the quantum wires are relativelythick and the mountainous parts 11 a between the V-groves are thin. Thestructure of this nature proves particularly effective when the shapesof V-grooves in the lower clad layer 12 or the guide layer 13 on thelower side are liable to dull. Notwithstanding this fact, even acontinuous active layer structure may well be regarded as a structure ofparallel disposed quantum wires so long as the region whichpredominantly exerts an effective carrier confining action concerningthe laser oscillation constitutes a relatively thick active layer part11 exclusively.

Further, the same function can be obtained and the oscillation with alow threshold can be achieved even when the width of the waveguideperpendicular to the direction Is of light emission are modulated alongthe direction Is of light emission in conformity with the period ofV-groove grating.

It has been found that the active layer itself does not need to begeometrically modified when the width of the waveguide mentioned aboveare modulated along the direction Is of light emission. This observationwill be explained with reference to FIGS. 2(A) and 2(B). A semiconductorlaser 10B of this invention illustrated in FIG. 2(A) is such that thepart of the active layer 15 serving as an effective oscillating part andhaving a predetermined width w15 does not jog and constitutes an activelayer 15 of the form of a uniform flat plane (sheetlike form). Theopposite sides thereof however, form the parts of mountain bases likethe bases of a mesa and are furnished with structures each havingV-grooves disposed in parallel along the direction Is of light emissionwith a period of nλ/4, such as ¼ or ¾, which is an integer times of thequarter wavelength λ in the medium, namely V-groove gratings. Asillustrated in FIG. 2(B) which depicts the active layer 15 and theopposite side part 15 a thereof exclusively, the opposite mountain baseparts 15 a continuing to the active layer 15 having the width w15mentioned above are so shaped as to give rise to grooves 15 bcorresponding to the V-grooves, while the effective part of the activelayer 15 which contributes to the laser oscillation is flat. It has beenfound that a periodic structure, such as of distribution of refractivityin the quantum well layer mentioned above, can be constructed with highaccuracy as expected and a low-threshold distributed feedback typesemiconductor layer can be likewise obtained even when the edge parts ofsuch an active layer are each provided with a structure of geometricallymodulated V-grooves.

This invention can utilize quantum dots as well. Also in thesemiconductor laser 10 illustrated in FIG. 1(A), the quantum wires 11depicted as assuming a falcate cross section, depending on theconditions of manufacture, may optionally be caused to form a pluralityof quantum dots within each of the V-grooves as though they were finelysplit. Naturally, even in this case, the same effect can be expected. Anartist's concept of this situation is depicted in FIG. 3. The quantumwires 11 which are depicted as a physical region continued in thedirection of width in the semiconductor laser 10 illustrated in FIG.1(A) assume a form as though split in the direction of width mentionedabove in a semiconductor laser 10C illustrated in FIG. 3 and constitutean aggregate of quantum dots 11′ in the individual grooves. Even in thisstructure, the same function and effect can be manifested as thesemiconductor laser of this invention illustrated in FIG. 1(A).

FIG. 4 illustrates a quantum nano-structure semiconductor laser 20according to another embodiment of this invention. This laser also usesquantum dots 21. When InGaAs is grown on such a V-groove substrate asdescribed already, with the increased In percent and the thickness ofgrown film and with a low growth speed, InGaAs or InAs multiple quantumdots 21 can be formed selectively on the V-grooves. The intervalsbetween the parallel disposed V-grooves, namely the intervals ofparallel disposition along the direction of advance of light between thegroups of quantum dots 21 arranged in a line, are set similarly to thecase of using the quantum wires described above at nλ/4, i.e. an integertimes a quarter wavelength in the medium of the laser active layer. Itis provided, however, that the shapes of the individual quantum dots 21are not infrequently dispersed as schematically illustrated in thediagram. And this fact possibly functions rather significantly asdescribed herein below.

Nevertheless, as is so in the case of the quantum wires 11 describedabove, the V-grooves of a limited length do not always retain the shapeof grooves incised in the substrate owing to the selective growth. Theyare allowed to use the shape which results from growing a lower cladlayer, forming a guide layer or the like, then withdrawing the substrateonce from a growing furnace, incising V-grooves of a limited lengththereon, and performing the second growth up to a plurality ofrepetitions as carried out in the conventional process, such as quantumwires and quantum dots, subsequently forming an upper guide layer and anupper clad layer. An attempt to retain the shape after the growth hasbeen made to a large thickness results in necessitating an enlargementto the period of grating. In the case of a grating using a period of notmore than 0.2 μm, the shape is preserved and grown only with difficulty.Such functions as allowing insertion of an active layer exclusively inthe anti-node of light are effectively manifested and the adherence tothe excellence of the V-groove shape of a substrate may be possiblydispensed with as occasion demands when the quantum wires having thelength thereof limited by the opposite closed terminals bring an effectof inhibiting dissipation of carriers and the quantum dots are formedexclusively at required places according to this invention.

These quantum dots 21 assume a structure such that the entire perimeterof an active layer is encircled with a clad layer of large bandgapenergy. Thus, they are equivalent in that buried heterostructures arerealized automatically Consequently, the dissipation of injectedcarriers can be prevented because the electron-hole pairs based on theinjection of electric current are retained within quantum dots asschematically shown with a thick arrow mark Ct in FIG. 4 and thediffusion length of minority carriers in the lateral direction along thedirection of width schematically indicated with fine arrow marks Ctbecomes small. A discussion regarding this effect of confinement itselfcan be found in Document 4: J. K. Kim, T. A. Strand, R. L. Naone, and L.A. Coldren, “Design Parameters for Lateral Carrier Confinement inQuantum Dot Lasers,” Appl. Phys. Letters, 74 (19) (May 10, 1999)2752-2754. When this effect is utilized, therefore, a distributedfeedback type quantum dot laser having a low threshold and enjoying astabilized oscillation frequency can be realized by a simple method ofproduction which merely comprises mesa-etching the stripe part of thestructure having groups of quantum dots formed thereon, therebytransforming the structure into a mesa structure 22 and obviates thenecessity for an embedded structure.

The material does not need to be particularly restricted, but is onlyrequired to allow realization of a structure contemplated by thisinvention. The InGaAs or InAs quantum dots that are formed by growingInGaAs or InAs to a thickness exceeding the critical film thickness onV-grooves are realistic. Commendably, the V-groove substrate is a GaAs(100) or (311)A substrate, or an InP (100) or (311)A substrate. Thispoint is held good with the other embodiments of this invention.

FIG. 5 illustrates another preferred embodiment 30 of the semiconductorlaser of this invention. According to the reports published to date, aquantum dot laser having quantum dots embedded in an in-plane patternhas been materialized by adopting a technique of crystal growth of theso-called SK mode (refer, for example, to Document 5: Z. Zou, D. L.Huffaker, S. Csutak, and D. G. Deppe, “Ground state lasing from aquantum-dot oxide-confined vertical-cavity surface-emitting laser,”Appl. Phys. Letters 75 (1), Jul. 5, 1999, p. 22).

A study is now underway regarding the manufacture of a DFB laser by theapplication of a metallic irregular surface grating or a selective ionimplantation with a focus ion beam to the lateral surfaces of awaveguide from the surface with the omission of an embedding re-growthprocess (refer, for example, to Document 6: H. Konig, S. Rennon, J. P.Reithmaier, and A. Forchel, “1.55 μm single mode lasers with complexcoupled distributed feedback gratings fabricated by focused ion beamimplantation,” Appl. Phys. Letters 75 (11), September 1999, p. 1491).

When a quantum well is used as an active layer, however, it is necessarythat the etch depth to define the ridge waveguide should be shallowabove the active layer for the purpose of avoiding carrier surfacerecombination on the processing interface. This shallow processing layerentails the problem of preventing acquisition of a fully satisfactorystabilization of wavelength.

According to this invention, however, it is made possible to suppressthe minority carrier diffusion velocity by forming a lower clad layer ona V-groove substrate satisfying the conditions defined by thisinvention, an active layer in a guide layer to confine therein groups ofquantum dots disposed with a prescribed period of an integer times ofthe quarter wavelength in the medium, and forming an upper clad layer asexplained above with reference to FIG. 3 Besides, as another structure,this invention allows construction of a DFB type optical resonator evenwhen the quantum dots themselves have random positions by forming alower clad layer 33 on a V-groove substrate 32, providing an activelayer in an upper-lower guide layer 34 with groups of quantum dots 31,thereby forming an upper clad layer 35, and boring the side of astructure confining the quantum dots 31 along the direction of advanceof light deep enough to permeate the quantum dots, thereby formingperiodic lateral surfaces as exemplified by a quantum nano-structuresemiconductor laser 30 illustrated in FIG. 5. That is, this inventionallows realization of a DFB laser having a low threshold and enjoying afully satisfactorily stabilized wavelength without requiring a re-growthprocess.

With reference to FIG. 5, one optical integrated circuit (OEIC) 39 isconstructed by integrally forming the part of the semiconductor laser 30nearly corresponding to the region having formed therein an electrode 37for a laser, serially with a modulator part 36 corresponding to theregion having an electrode 38 for a modulator formed therein. Thestructure of this modulator part 36 per se is not particularlyrestricted by this invention, but may be a proper known structureselected arbitrarily.

As mentioned above, since the advent of the so-called “Fiber to theHome” age, the need of freely controlling the optical signals on anoptical fiber network has been urging attention and the necessity formanufacturing semiconductor laser arrays of different frequencies withthe object of freely switching and exchanging the lights emitted bythese arrays has been finding growing recognition. FIG. 6 is a schematicview of an optical integrated circuit 49 using a two-dimensionalphotonics crystal and constructed in accordance with this invention.This circuit 49 is constructed by forming a lower clad layer 43 on asemiconductor substrate 42, disposing thereon in accordance with thisinvention as interposed between upper and lower guide layers amultiplicity of groups of quantum dots with a prescribed period, namelya period of an integer times the quarter wavelength in the mediumpreferably as further superposed in a plurality of layers in thedirection of height, forming an upper clad layer 45 a and a surfacelayer 45 b, and forming an electrode 47 for a laser in a laser regionpart 40.

The embodiment illustrated in FIG. 6 is characterized by having holes 46bored through a structure having superposed quantum dot layers formedtherein along the lateral sides thereof so as to leave behind stripeparts for passing a laser beam in the direction of advance of light andin the vertical direction perpendicular to the lateral directionperpendicular to the direction of advance of light with a prescribedperiod. The period is set, for example, at ½ of the wavelength in themedium.

Consequently, the stripe parts enclosed with the lines of the holesextending along the direction of advance of light constitute waveguides.By designing the intervals of these holes, therefore, it is madepossible to allow the waveguide wavelength selectivity and enable asignal to be guided through specific waveguides.

That is, the OEIC 49 necessary for an optical transmission system can berealized by having lasers, modulators, branches, filters, etc. connectedonto one and the same substrate through the two-dimensional photonicsoptical waveguides. In the illustrated case, the modulators providedwith the electrodes 48 for modulation are integrated.

As observed in this embodiment, the fact that an active optical modulecan be realized only by the disposition of holes and electrodes ishighly valuable even practically.

When the quantum dots are used, since the surface recombination poses nobig problem, a two-dimensional optical circuit can be formed by anadditional fabrication. Incidentally, the holes 41 may be empty holesfilled with air and these empty holes may be optionally filled with amedium of proper refractivity. They are only required to serve as emptyholes from the viewpoint of an optical circuit.

FIG. 7 depicts a monolithic OEIC 50 that makes use of this invention. Ithas on a proper substrate a ridge type optical waveguide 51 which may beconstructed by any of the known techniques. This waveguide 51 has formedtherein branch lines that severally serve properly as opticalwaveguides. These branch lines are each required to have a semiconductorlaser of this invention constructed in advance therein as alreadyexplained above with reference to FIGS. 1 to 3. It is schematicallydepicted, with a portion magnified for the sake of explanation. Thequantum wires 11 already explained and the quantum dots 11′, the activelayer 15 and the like described above are disposed in the semiconductorlaser region 10 (or 10A, 10B, 10C). Thus, the OEIC 50 constitutes asemiconductor laser array. Otherwise, by varying the conditions ofperiods concerning the relevant dispositions relative to the wavelengthin the medium and by varying the construction and parameter of the ridgetype waveguide, it can be utilized as an integrated multi-wavelengthlight source. Actually, the ridge type optical waveguide may be formedafter semiconductor laser parts are formed at proper locationspreliminarily in the V-groove grating structure under the conditions inconformity to this invention. This invention produces the ease and theconvenience with which such structures are formed un V-grooves by onecycle of selective growth.

Of course, it is possible to provide each of these waveguides with thestructure of an active layer region that is used in the semiconductorlaser of this invention as already described with reference to FIGS. 3to 6.

This invention is enabled by taking into due consideration the samedispositional relation as in the quantum nano-structure semiconductorlaser which has been described hitherto concerning the disposition oflimited-length quantum wires or quantum dots to provide not onlysemiconductor lasers but also quantum nano-structure arrays which areapplicable to various “optically functional elements.” In other words,it promises useful applications when this invention is defined as alimited-length quantum wire array or a quantum dot array confined in thewaveguide region for passing light within the width thereof and paralleldisposed in the direction of advance of light with a period of aninteger times of the quarter wavelength in the medium of the waveguide.The materials and the procedure to be adopted for the manufacture ofthis array may be the same as those described hitherto regarding thequantum nano-structure semiconductor laser of this invention.

By the quantum nano-structure array contemplated by this invention,namely by the structure having limited-length quantum wires or quantumdots disposed with a period conforming to this invention, since thedensity of states is rendered discrete and the width of gain band isnarrowed in the semiconductor laser, the injected carriers areefficiently concentrated on a quantum level conforming to a specificwavelength of oscillation. And this concentration results in decreasingthe threshold. When it is applied directly to the active layer of amodulator as one of the optically functioning elements, for example,sharp state density means that the gain is peaked at a sharp wavelengthcharacteristic of the Lorentz type and results in suppressing the changeof the peak position thereof

The wavelength dependence of refractivity corresponds to thedifferential waveform. As a result, the refractivity is nullified in theneighborhood of the center of the gain being oscillated and stableagainst the variation of carrier concentration. Generally, change ofrefractivity against the carrier density is called an “alpha parameter.”The active layer formed of the limited-length quantum wire array orquantum dot array disposed in conformity with this invention is capableof appreciably decreasing the alpha parameter and, therefore, is enabledto effect high-speed modulation.

Even when external modulation is resorted to the use of the quantumnano-structure array conforming to this invention results in decreasingthe time delay of the individual wavelengths due to the variation ofbias in accordance with the steep absorption characteristics. Thus, byharnessing the difference of slope on the opposite sides of theabsorption characteristics, it is made possible to efficiently executethe manipulation of wavelength dispersion, such as of delivering theshort wavelength component having a slow speed of transmission ahead ofother components.

The application of the quantum nano-structure array of this invention tothe active layer of an optical amplifier, as clearly inferred fromformer embodiment described with reference to the schematic view of FIG.4, can be actually expected to widen the range of the presence of gains(about 100 nm, for example) due to the inherent size fluctuation ofquantum dots. Thus, it becomes feasible to amplify channels spread overa wide zone collectively.

The mode lock laser and the Mach-Zender type optical switch utilize asupersaturated absorber and the speeds of response of these devices justmatch the frequency zone necessary for the optical transmission. To bespecific, the optical transmission utilizes the pulse transmission in anapproximate frequency range of 10 GHz to 100 GHz, which corresponds tothe time domain of 10 ps to 100 ps. Thus, such a relaxation phenomenonas enables restoration to the former state within several ps provesadvisable. In the case of quantum dots, the relevant pulse width justreaches the range of picoseconds because the capture of multiplecarriers in the dot level, is slower than the quantum well. If thecapture is unduly fast, the number of photons necessary for saturationwill be wasted. If it is unduly slow, the restoration to the initialstate will not be finished till the next pulse arrives. The speed justfalls within the proper range of several pico-second for opticalcommunication. Thus, the adoption of a limited-length quantum wire arrayor quantum dot array constructed by the disposition of this inventionenables this control to be effectively executed.

The quantum nano-structure array constructed by this invention can beeffectively applied as a frequency modulator/converter. When thethree-dimensional nonlinear effect is used and a pump light and a signallight are injected, the frequency conversion is effected at the angularfrequency ω_(c) following the formula; ω_(c)=ω(pump light)×2−ω(signallight). Since this effect is produced by the coherent interaction of thelight and the quantum state in the quantum nano-structure, it occursmore strongly when the quantum state is not perturbed from the exteriorand the phase state is retained than otherwise. The phase relaxing timeis called “gamma (γ).” The quantum dots having an isolated quantum statehave a longer phase relaxing time and, therefore, manifest the nonlineareffect strongly. Since this invention provides a quantum nano-structurearray which has the limited-length quantum wires or quantum dotsarranged periodically, it enables the wires or dots to concur on theanti-node of light, causes the aforementioned effect to manifeststrongly in a small volume of the active layer, delays the groupvelocity of light (in other words, reciprocation of light in thematerial) and allows the gain and absorption of light to occurefficiently.

In short, the quantum nano-structure array of this invention can serveas an effective device also in constructing various opticallyfunctioning elements mentioned above.

In all the embodiments described hitherto, the quantum nano-structuresemiconductor lasers and the quantum nano-structure arrays alike whichconform to this invention have been delineated as having limited-lengthquantum wires parallelly disposed, quantum dots formed therein andV-grooves forming an active layer thereon parallel disposed invariablywith a period of an integer times of the quarter wavelength in themedium. By intentionally displacing them from the period of an integertimes of the quarter wavelength in the medium of the laser active layeror the waveguide layer, it is made possible to stabilize the waveguidemode. Then, by promoting the dispersion compensation among theoscillation modes within the active region in the case of asemiconductor laser or by applying the dispersion compensation againstthe passing light in the case of an externally disposed mirror orwaveguide, it is made possible to attain broadband wavelengthoscillation or short pulse oscillation in the mode lock mechanism.

Regarding the latter compensation of dispersion, a commendable techniquefor optimizing this compensation has been already disclosed in JP-A2000-352614. When the construction contemplated by this invention isrelied on to promote the compensation of dispersion, therefore, theinvention already disclosed as described above may be consulted indeciding the degree of the displacement mentioned above. To cite onesheer example, since the dispersion of wavelengths (difference in speedof light propagation due to difference in wavelength) can be varied forindividual wavelengths depending on the degree of the displacement ofthe period of quantum wires or V-grooves from ¼ of the wavelength in themedium, it is made possible by causing numerous longitudinal modes to bepropagated invariably at the same speed to induce formation of a statein which numerous wavelengths are synchronized in the same phase (modelock) or to induce efficient generation of a light pulse of an extremelyshort (2-30 fs) width.

INDUSTRIAL APPLICABILITY

According to this invention, there can be provided a semiconductor laserexcelling in the property of stabilizing the oscillation frequency witha low threshold preferably by one cycle of selective growth as describedabove. Since the invention enables limited-length high-density multiplequantum wires and quantum dots to be formed at any necessary positionsin the structure of a device, a quantum nano-structure array whichpromises significant application to highly sophisticated quantumnano-structure semiconductor lasers and various optically functioningelements can be realized by a simple manufacturing process.

Further, the communication wavelength zone tends to be enlarged in thefuture and the wavelength zone of 1.0-1.6 μm has been gaining inimportance. The use of the quantum nano-structure array (limited-lengthquantum wire array and quantum dot array) provided by this inventionallows the wavelength range to be enlarged. Since the array obviates thenecessity for re-growth and permits use of a mixed crystal incorporatingan Al composition of a large band gap as a clad layer, it is madepossible to manufacture a laser not easily affected by the operatingtemperature (endowed with a high To) and materialize a laser of astabilized wavelength which is suitably applied to products for generaluse, such as household appliances and automobiles, which allow rigidcontrol of temperature conditions only with difficulty.

1. A quantum nano-structure semiconductor laser comprising: a groovedsemiconductor substrate having a plurality of V-grooves individuallyextended in directions perpendicular to a direction of advance of anoscillated laser beam and mutually disposed in parallel along thedirection of advance of the laser beam; and a plurality of quantum wiresformed one on each of the V-grooves by selective growth of a Group III-Vcompound, said plurality of quantum wires being disposed in parallelalong the direction of advance of the laser beam with a period of aninteger times of a quarter wavelength in a medium of a laser activelayer and individually closed at both ends thereof to form an activelayer region of a limited length, which length is equal to a stripewidth of the laser.
 2. A quantum nano-structure semiconductor laseraccording to claim 1, wherein a waveguide mode is stabilized byintentionally displacing a period of parallel disposition from saidperiod of an integer times ¼ instead of disposing said plurality ofquantum wires in parallel along said direction of advance of the laserbeam with a period of an integer times of the quarter wavelength in themedium of the laser active layer for a broadband wavelength oscillationor short pulse oscillation in a state of mode lock is materialized bypromoting compensation of dispersion between oscillation modes.
 3. Aquantum nano-structure semiconductor laser according to claim 1 or claim2, wherein said substrate is a GaAs (100) or (311)A substrate; saidV-grooves are limited-length V-grooves formed in a [01-1] direction onthe GaAs (100) or (311)A substrate; said quantum wires arelimited-length wires manufactured from GaAs or InGaAs and grown on saidlimited-length V-grooves; and said quantum wires are individuallyfurnished with GaAs or AlGaAs clad regions adapted to cover said quantumwires.
 4. A quantum nano-structure semiconductor laser according toclaim 1 or claim 2, wherein said substrate is an InP (100) or (311)Asubstrate; said V-grooves are limited-length V-grooves formed in a[01-1] direction on the InP (100) or (311)A substrate; said quantumwires are limited-length wires manufactured from InGaAs and grown onsaid limited-length V-grooves, and said quantum wires are furnished withInAlAs clad areas adapted to cover said quantum wires.
 5. A quantumnano-structure semiconductor laser comprising: a grooved Group III-Vcompound semiconductor substrate having a plurality of V-groovesindividually extended in directions perpendicular to a direction ofadvance of an oscillated laser beam and mutually disposed in parallelalong said direction of advance of the laser beam with a period of aninteger times of a quarter wavelength in a medium of a laser activelayer; a plurality of InGaAs or InAs quantum dots formed one in each ofbase parts of said V-grooves by growing InGaAs or InAs on said V-groovesto not less than a critical film thickness, said plurality of quantumdots forming an active layer region of a limited length, which length isequal to a stripe width of the laser; and GaAs or AlGaAs layers on saidactive layer region serving as clad regions.
 6. A quantum nano-structuresemiconductor laser according to claim 5, wherein a waveguide mode isstabilized by intentionally displacing said period of paralleldisposition from said period of an integer times ¼ instead of disposingthe period of parallel disposition of said plurality of V-grooves at aninteger times of the quarter wavelength in the medium of said laseractive layer, for a broadband wavelength oscillation or short pulseoscillation in a state of mode lock is materialized by promotingcompensation for dispersion between oscillation modes.
 7. A quantumnano-structure semiconductor laser according to claim 5 or claim 6,wherein said grooved Group III-V compound substrate is a GaAs (100) or(311)A substrate or an InP (100) or (311)A substrate.
 8. A quantumnano-structure semiconductor laser according to claim 5 or claim 6,wherein a structure having said quantum dots formed therein ismesa-etched to be transformed into a distributed feedback laser.
 9. Aquantum nano-structure semiconductor laser according to claim 5 or claim6, wherein a structure having said quantum dots formed therein haslateral surfaces along said direction of the laser beam imparted withcorrugations to be transformed into a distributed feedback laser.
 10. Aquantum nano-structure semiconductor layer according to claim 5 or claim6, wherein a structure having said quantum dots formed therein has aplurality of through holes bored therein with a prescribed period alonglateral sides leaving behind stripe parts for passing light and alongvertical directions individually perpendicular to both said direction ofadvance of the laser beam and a lateral direction perpendicular thereto.11. A quantum nano-structure semiconductor laser according to claim 10,wherein said period is ½ of the wavelength in the medium.